The present invention particularly relates to a material holder used in a differential scanning calorimeter of the type disclosed and claimed in U.S. Pat. No. 3,732,722 (“the '722 patent”) for a “Material Holder” issued May 15, 1973 to Norem et al. and assigned to The Perkin-Elmer Corporation of Norwalk, Conn. The entirety of the '722 patent is incorporated by reference herein.
A differential scanning calorimeter is a thermal analytical instrument which operates on the principle that thermal energy is absorbed or evolved during physical or chemical changes in a material which is being analyzed. The differential scanning calorimeter measures the differential energy changes that occur in a sample material as compared to a reference material, during such physical or chemical changes. The sample material and a thermally inert reference material are placed in separate material holders in the same thermal environment and their temperatures are measured during the analysis process.
The material holder, as disclosed in the '722 patent, includes a cylindrical cup having three bottom partitions forming two cavities within the bottom portion of the cup, one of the cavities containing a heating winding, and the other cavity containing a heat-sensing winding. A supporting center post is attached to the bottom of the lowermost partition as a support for the material holder.
In order to produce the material holder, a series of substantially identical discs are punched from sheet stock with a punch and die set to form the three partitions. These discs are successively press fitted into a cylindrical housing and electron beam welded at the disc edges to the cylindrical housing. The heater and sensing windings are sandwiched between the successive disc partitions.
Serious problems have been encountered in the above-described assembly and fabrication method in that it has been found to be extremely difficult to maintain tolerances on the disc diameters and the inside diameters of the cylindrical housings with sufficient precision to avoid very high rejection rates in the assembled material cups. The success of the instrument resides in maintaining extreme precision. The precision in measurement is so important that a noble metal such as platinum, paladium, gold, and their alloys are preferably used as the material of the material holder. Preferably, the metal of which the holder is formed is an alloy of 80% platinum and 20% iridium. The noble metals avoid the problems of oxide formation.
The high precision also requires extremely narrow tolerances in the assembled dimensions of the material holders. Thus, in a material holder which is on the order of one-third of an inch in diameter (actually 0.360 inches in one preferred embodiment), the various partitions must be perfectly parallel with one another with a variation out of parallelism not to exceed 0.001 inch. Also, the various partitions must not be distorted in shape, the compartments containing the windings must be absolutely consistent and uniform in size, within the tolerance of plus or minus seven ten thousandths of an inch, and there must be very good conductivity between the edges of the partition discs and the cylindrical housing. In order to accomplish these purposes, the partitions must be press fitted into position within the cylindrical housing with a tight enough fit to precisely maintain the position of each disc during handling after assembly and before electron beam welding. Furthermore, the press fit must be sufficiently tight to promote the production of a good sound electron beam weld between the disc partition and the cylindrical housing in order to provide a consistently high thermal conductivity through the joint formed thereby. On the other hand, the disc must not be press fitted with so much of an interference fit that it results in distortion of the disc.
One of the biggest problems in achieving the satisfactory press fit of the disc partitions into the cylindrical housing apparently arises because of variations in the diameters of the discs produced by the punch and die set. For a particular batch run of approximately 900 discs between sharpenings of the punch and die set, it has been discovered that the disc diameters may vary in a typical range of about {fraction (3/10,000)}ths of an inch, or more. These variations in disc diameter are believed to be associated with the wear pattern of the punch and die, and also, possibly, these variations may be related to variations in the toughness and thickness of the sheet metal from which the discs are punched.
Another problem in providing the correct press fit arises particularly in connection with the bottommost disc partition to which the support post is welded. It has been discovered that the support post welding causes the disc to which it is welded to shrink somewhat in diameter, that shrinkage being in the order of 4 to 5 ten thousandths of an inch.
Another major problem in producing a satisfactory press fit of the disc partitions has been found to result from the fact that the electron beam welding of the first (uppermost) partition into the cylindrical housing causes the diameter of that housing to shrink slightly, so as to increase the tightness of the cylinder around the edges of subsequently assembled disc partitions. Similarly, the electron beam welding of the second disc partition causes still a further shrinking of the cylindrical housing for the assembly of the third disc partition.
These problems were substantially obviated by U.S. Pat. No. 4,330,933 (“the '933 patent”) for a “Fabrication Method For Instrument Material Holder” issued May 25, 1982 to Bullinger et al. and assigned to The Perkin-Elmer Corporation of Norwalk, Conn. The entirety of the '933 patent is incorporated by reference herein.
The '933 patent teaches an improved fabricating method which compensates for at least part of the fluctuations in disc diameters, and which compensates for the shrinkage of the cylindrical housing during electron beam welding of the first disc which includes the steps of sorting the discs according to diameter into at least two sets including a first set and another set having diameters smaller than the discs of the first set. Cylindrical housings are then machined with an inside diameter somewhat smaller than the outside diameters of the discs of the first set to provide a press fit of the discs of the first set within the housings. After press fitting and electron beam welding of a disc from the first set within each cylinder housing, one of the discs from the smaller diameter set is press fitted into the housing, the smaller diameter of the disc compensating for the shrinkage of the cylinder caused by the welding of the first disc.
In another aspect of the invention disclosed in the '933 patent, an improved fabrication method may be provided in which the discs are divided into at least two sets, including a first set and another set of discs which are larger in diameter than the discs of the first set. A batch of cylindrical housings are then machined with an inside diameter to provide a desired press fit of the discs of the first set within the housings. After a disc from the first set is press fitted into a cylindrical housing and welded therein, one of the larger diameter discs, to which a supporting center post has already been welded, is press fitted into the end of the cylindrical housing. The reduction in the diameter of the last mentioned disc caused by the welding of the center post compensates for the larger initial diameter of the disc and the reduction in the diameter of the cylindrical housing caused by the welding of the earlier disc into the housing.
The above mentioned fabrication processes may be combined into a single process in which the initial run of discs is sorted by diameter into a first set, another set of discs having a diameter smaller than the discs of the first set, and still another set of discs having a diameter larger than the discs of the first set, the discs of the first set being used as the first disc partition to be positioned and welded into each cylindrical housing, the inside diameter of which has been machined to provide the appropriate press fit, the smaller diameter discs being used as the second partition to be press fitted and welded into each housing, and the larger diameter discs being the ones to which supporting posts are attached prior to press fitting and welding into the cylindrical housing to provide the third partition.
While the improved fabrication processes disclosed in the '933 patent obviate many of the problems with the processes disclosed in the '722 patent in that the fabrication processes disclosed in the '933 patent avoid the problems arising from variations in the disc diameters as produced by the punch and die, avoid the problem associated with the shrinkage in the diameter of the bottommost disc partition occasioned by the welding of the support post thereto, and avoid the consequences of the problem of the shrinkage of the cylindrical housing resulting from the electron beam welding of earlier assembled disc partitions in order to improve the press fit of subsequently assembled disc partitions, the fabrication processes disclosed in the '933 patent suffer from disadvantages of their own.
More specifically, the processes disclosed in the '933 patent are very labor intensive to ensure proper spacing, requiring the use of precision tooling fixtures and pins. Even when the operator is extremely careful, it difficult to accurately control the spacing of the heater and the sensor, and it is difficult to achieve reproducibility and consistent performance characteristics between material holders. Moreover, the material holders disclosed in the '933 patent are comprised of a relatively high number of parts, and result in a high rate of scrap material (on the order of 40% to 50%) due to the fact that the heater and sensor assemblies are assembled to the center section from each end. Furthermore, using the processes disclosed in the '933 patent, it is difficult to accurately know where to place the weld with the electron beam welder, which can result in spluttered platinum in the open cavities.
What is desired, therefore, is a holder for materials used in measuring instruments and a fabrication method therefor which requires less labor intensive processes than are currently employed, which ensures proper spacing without requiring the use of precision tooling fixtures and pins, which allows for the spacing of the heater and the sensor to be accurately controlled, which facilitates reproducibility and consistent performance characteristics between material holders, which has a reduced number of parts, which has a reduced scrap rate, and which facilitates the accurate placement of the electron beam weld so as to inhibit spluttered platinum in the open cavities.